The present invention relates to assays for an analyte, such as an antigen, in a liquid sample, such as body fluid. More particularly, the present invention relates to a method and device for the detection of an analyte in a body fluid using a lateral flow test cell containing a novel biphasic chromatographic substrate.
Many types of ligand-receptor assays have been used to detect the presence of various substances in body fluids such as urine or blood. These assays typically involve antigen-antibody reactions, synthetic conjugates comprising enzymatic, fluorescent, or visually observable tags, and specially designed reactor chambers. In most of these assays, there is a receptor (e.g. an antibody) which is specific for the selected antigen, and a means for detecting the presence and/or amount of the antigen-antibody reaction product. Most current tests are designed to make a quantitative determination, but in many circumstances all that is required is a positive/negative indication. Examples of such qualitative assays include blood typing, pregnancy testing and many types of urinalysis. For these tests, visually observable indicia such as the presence of agglutination or a color change are preferred.
The positive/negative assays must be very sensitive because of the often small concentration of the ligand of interest in the test fluid. False positives can be troublesome, particularly with agglutination and other rapid detection methods such as dipstick and color change tests. Because of these problems, sandwich assays and other sensitive detection methods which use metal sols or other types of colored particles have been developed. These techniques have not solved all of the problems encountered in these rapid detection methods, however. It is an object of the present invention to provide an improved detection device and method having greater sensitivity and discrimination for analytes of interest. Another object of the invention is to provide an assay device which is simpler to manufacture.
The present invention provides a rapid, sensitive device and method for detecting the presence of analytes in body fluids. The method and device have high sensitivity and result in virtually no false positives. Use of the present device and method provides an assay system which involves a minimal number of procedural steps, and reproducibly yields reliable results even when used by untrained persons.
The device and method utilize a unique biphasic chromatographic medium which enhances the speed and sensitivity of the assay. According to the present invention, a biphasic substrate element is provided comprising a release medium joined to a capture medium located downstream of said release medium. The release and capture media preferably comprise two different materials or phases having different specific characteristics. The two phases are joined together to form a single liquid path such that a solvent front can travel unimpeded from the proximal (upstream) end of the release medium to the distal (downstream) end of the capture medium.
The release medium comprises a bibulous, hydrophilic material, such as absorbent paper. Preferred materials for use as a release medium include cotton linter paper, cellulosic paper, or paper made of cellulose together with a polymeric fibrous material, such as polyamide or rayon fibers, and glass fiber material. The primary function of the release medium is first to support and to subsequently release and transport various immunological components of the assay, such as a labeled binding member and a capturable component, both of which have specific affinity for the analyte of interest. This release and transport occurs during routine operation of the assay.
The capture medium comprises a hydrophilic polymeric material, preferably a nitrocellulose or nylon membrane. The preferred materials for use as a capture medium are microporous films or membranes which permit protein reagents to be immobilized directly on the membrane by passive adsorption without need, for chemical or physical fixation. For this purpose, membranes of nitrocellulose, nylon 66 or similar materials are preferred most preferably having a pore size in the range of from about 5 xcexcto about 20 xcexc. The nitrocellulose membrane may be nitrocellulose alone or a mixed ester of nitrocellulose. The nitrocellulose membrane preferably is coated or laminated onto a translucent or transparent polymeric film to provide physical support for the membrane. In a currently preferred embodiment, a nitrocellulose polymer which has been cast onto a polyester film such as Mylar(copyright) is used. Alternatively, a nitrocellulose membrane laminated onto a polyester film also may be used. Other bacidng materials besides polyester may be used. The primary function of the capture medium is to immobilize an immunological or chemical affinity agent at one or more capture sites for capturing the reagents released from the release medium.
As stated above, the release and capture media are joined together to form a single liquid path. Reagents for detecting labeling and capturing the analyte of interest are disposed on the release and capture media. Located on the release medium is a binding member reactive with a first epitope of the analyte of interest. The binding member is labeled with a detectable marker. A capturable component is located on the release medium downstream of the binding member, which component comprises a binding agent reactive with a second epitope of the analyte and one member of an affinity pair. The capturable component is capable of forming a complex with the labeled binding member and the analyte. The labeled binding member and the capturable component both are releasably bound to the release medium such that when the solvent front created by the liquid sample being analyzed passes through the release medium, the labeled binding member and the capturable component both become solubilized by the liquid and flow with the solvent along the liquid path. In operation, if any analyte is present in the liquid sample, it reacts first with the labeled binding member, then with the capturable component as the front advances along the liquid path. By the time the solvent front reaches the capture medium section of the biphasic material, the capturable complex has formed.
The capture site located on the capture medium comprises the other member of the affinity pair specific for the capturable component. The affinity member is immobilized, preferably by simple adsorption, at the capture site, and does not advance with the solvent front.
In a preferred embodiment, a control site also is located on the capture medium downstream of the capture site. The control site has immobilized thereon a binding agent having an affinity for the labeled binding member. The binding agent will capture any labeled binding member which is not captured at the upstream capture site. In operation, the presence of the detectable marker at the control site indicates that sorptive transport has operated properly.
The present invention further provides a device for detecting the presence of an analyte in a liquid sample. The device comprises an elongate casing housing the biphasic medium, and defining a liquid sample inlet, a reservoir volume, a test volume interposed between the inlet and reservoir volume, and a window through the casing to observe the test result. Preferably, the sample inlet and the window are located in opposite sides of the casing. The casing is adapted to receive the assay materials, which are disposed on the biphasic medium sequentially within the casing. The assay materials comprise an optional sample absorbent, the biphasic chromatographic substrate and a reservoir absorbent. The chromatographic medium is positioned within the casing such that the capture site, and the control site if applicable, are visible through the window. The sample absorbent, biphasic chromatographic substrate and reservoir absorbent are in fluid communication and together form a liquid path.
In a currently preferred embodiment, the device comprises a casing defining a sample inlet, a test volume and reservoir volume. Disposed within the casing are a sample absorbent, the biphasic chromatographic substrate and reservoir absorbent. The sample absorbent is disposed within the casing opposite the sample inlet. Located downstream of the sample absorbent is the biphasic chromatographic substrate comprising a release medium and a capture medium joined together to form a single liquid path. The release medium preferably comprises sorbent paper, and the capture medium preferably comprises nitrocellulose membrane. The release and capture media preferably are both laminated onto a transparent plastic film or sheet. Disposed on said release medium is (i) a binding member comprising a specific binding protein, e.g., a monoclonal antibody reactive with a first epitope of said analyte, said antibody being labeled with a visually detectable marker such as colloidal gold particles; and (ii) a capturable component comprising a biotinylated binding protein, e.g., an antibody preferably disposed downstream of said labeled antibody. The biotinylated antibody is reactive with a second epitope of the analyte and is capable of forming a complex with the labeled antibody and the analyte. Disposed on the capture medium is a capture site for capturing and immobilizing the complex. The capture site has immobilized thereon a capture component which has a high affinity for the biotin portion of the complex, preferably streptavidin.
The biphasic chromatographic medium preferably further comprises a control site disposed on the capture medium downstream of said capture site. The control site has immobilized thereon an agent capable of capturing said labeled antibody. The primary function of the control site is to capture and immobilize labeled antibody which has not been captured at the capture site. In the currently preferred embodiment, the control site has immobilized thereon polyclonal antisera specific for the labeled antibody. The appearance of color from the gold particles at the control site indicates proper functioning of the test, irrespective of the presence or absence of analyte in the sample. Both the capture and control sites must be visible through the window of the casing.
In the method of the invention, the proximal end of the biphasic substrate is contacted with the liquid sample being analyzed. The liquid sample travels impelled by surface effects such as by capillary action along the liquid path formed by the substrate. If the analyte of interest is present in the sample, it sequentially reacts with the labeled binding member and the capturable component, forming the capturable complex, followed by reaction of the complex with the immobilized capture component at the capture site. This process results in the labeled complex accumulating at the capture site. The presence of the analyte is determined by observing the presence of the detectable marker at the capture site. If no analyte is present in the sample, the capturable complex does not form and no detectable marker will be present at the capture site. If a control site is present, the unbound complex or the free labeled binding member will accumulate at the control site.
The method of the invention also may be designed to exploit conventional xe2x80x9csandwichxe2x80x9d or xe2x80x9ccompetitivexe2x80x9d techniques. In the case of the sandwich technique, the labeled binding member comprises an antibody which binds to an epitope on the analyte of interest to form a labeled antibody-antigen complex. This complex then migrates to the capture site to react with a capturable component which, in this embodiment, comprises a second antibody specific for a second epitope of said analyte. For example, in the case of biotin, the affinity member may be streptavidin. At the capture site, the analyte and labeled antibody reacts with the immobilized capture member to form a xe2x80x9csandwichxe2x80x9d of the second antibody, analyte and labeled antibody. This sandwich complex is progressively produced at the capture site as sample continuously passes by. As more and more labeled conjugate is immobilized at the capture site, the colored particles aggregate and become visible through the window of the casing, indicating the presence of the analyte in the liquid sample. Both in the presence or absence of a detectable level of analyte, the colored particles gather at the control site which also is visible through the window.
In the case of the competitive technique, a known amount of the analyte of interest is present on the release medium disposed upstream of an antibody specific for it. The analyte present in the release medium is labeled. The labeled analyte on the release medium may comprise, for example, an authentic sample of the analyte, or a fraction thereof which has comparable affinity for the antibody. As the liquid sample is transported along the release medium, the labeled analyte present on the release medium and any unlabeled analyte present in the sample compete for sites of attachment to the antibody. If no analyte is present in the sample, labeled analyte-antibody aggregates at the capture site, and the presence of color indicates the absence of detectable levels of analyte in the sample. If analyte is present, the amount of labeled analyte which binds at the test site is reduced because of binding of analyte in the sample with the antibody, and no color, or a paler color, develops.
Alternatively, the system described for xe2x80x9csandwichxe2x80x9d assay may be used. The antibody specific for the analyte is biotinylated, with steptavidin being immobilized at the capture site.
The use of the colored particle detection system in combination with the unique biphasic substrate enables construction of a family of extremely sensitive assay systems which minimize the occurrence of false positives and which can be used effectively by untrained persons.